1. Field of the Invention
This invention relates to the preparation of alumina coated structures, and more particularly to a method of preparing alumina coating solutions and regenerating depleted alumina coating solutions.
2. The Prior Art
A catalyst and a catalyst support comprising a substrate having an adherent film of alumina formed thereon is disclosed in coassigned U.S. Pat. No. 3,231,520, incoporated herein by reference. The alumina film is disclosed as being useful as a support for catalytic materials which promote various reactions, such as those useful in treating exhaust gases from internal combustion engines. The alumina film is disclosed as being formed on a substrate, which can be metallic or nonmetallic and which can have a variety of configurations, by first contacting the substrate with a solution of an alkali metal aluminate. The adherent film of hydrated alumina which is formed on the substrate is then dried and calcined, producing a hard tenacious film of alumina on the substrate.
Coassigned U.S. Pat. Nos. 3,227,659 and 3,410,651 disclose the impregnation of the calcined alumina film with phosphorus and chromium containing materials, respectively. Coassigned British Pat. No. 1,271,710 discloses that the alumina coated substrate can be used by itself, without any added materials, as a filter to remove lead or lead compounds. The use of an alumina coated substrate to filter and remove carbon particles from diesel engine exhaust gases is disclosed in coassigned U.S. Pat. No. 4,039,294.
In coassigned U.S. Pat. No. 3,231,520 the preferred sodium aluminate coating solution, used for applying the adherent alumina film, is disclosed as being formed by dissolving metallic aluminum in a relatively strong aqueous solution of sodium hydroxide. Alternatively, when deemed desirable, the coating solution can also be formed by dissolving alumina in a strong aqueous solution of sodium hydroxide, or dissolving commercial sodium aluminate in water. A substrate is then immersed into the coating solution. During a period of time, which varies depending on the temperature of the solution and the concentration of sodium aluminate and sodium hydroxide in the solution, the hydrated alumina film forms on the substrate and grows until it reaches a desired thickness.
In many previously used coating methods, excess aluminum metal was added to the sodium hydroxide solution. As the alumina film formed on the substrate, the concentration of the sodium aluminate in the solution decreased, and aluminum metal dissolved into the sodium hydroxide solution to maintain an almost constant supersaturated solution of sodium aluminate.
French Pat. No. 75.39408 notes certain difficulties encountered in using aluminum metal to form the coating solution. Highly pure aluminum metal, of 99.99 percent purity, is preferred in such processes, since aluminum metal of a lower purity can give a violent reaction when dissolved into the sodium hydroxide solution. This makes the rate of dissolution of the aluminum metal into the sodium hydroxide solution, and the resultant continuous coating of the alumina, difficult to control. The difficulty in controlling the reaction results in difficulty in regulating or controlling the speed of deposition of the alumina layer on the substrate. For these reasons, the use of impure aluminum metal may result in undesirably long coating times. Further, the evolution of hydrogen gas, which accompanies the use of aluminum metal, is a cause of concern in commercial applications of the coating process. The use of high purity aluminum metal is objectionable because of the higher cost of such pure metal.
The French patent discloses a method in which the aqueous solution of alkali aluminate is formed by dissolving aluminum oxide in sodium oxide solution. Deposition of the alumina on a substrate is accomplished by small pH changes in the coating process. However, the process requires a complicated multistep process and results in long coating times, upwards of 40 hours.